Torn Skin

Like many people, I have been absorbed in the news reports about the horrifying triple-tragedy in Japan — first, an enormous earthquake, followed by a devastating tsunami, and now this continuing crisis with the damaged nuclear power plant. The available information on this latter topic has been sketchy, at best. Some people are blaming the Japanese government or the power company for a “lack of transparency”. I don’t think it’s appropriate to be pointing fingers, considering the overwhelming problems they are facing, but for my own satisfaction I have been trying to piece together a clearer picture of what went wrong, and how these incidents might be prevented in future. After all, the nearest nuclear plant to my location is also situated on the edge of the ocean, near an active fault, so the question is all too germane.

Keeping cool

All the issues at the Fukushima Daiichi plant can be related to a single factor: cooling, or rather, lack of same. A fission plant generates electricity in pretty much the same way as a natural gas or coal plant — it boils water, and the steam is used to turn a turbine, which spins a generator. The difference is,  in the fossil fuel plants, if you shut off the gas valve, or stop shoveling coal, the fire goes out, the water cools down, and you can pretty much walk away. But nuclear fuel rods generate heat via their natural radioactivity — you can dampen the reactor core to shut down the chain reaction, but the fuel stays hot. In the reactors used at Fukushima, the core will be boiling off 60 gallons of water per minute one week after shutdown. So it is imperative that a continuous supply of cool water is circulated through the reactor in order to draw off this heat, for more-or-less the entire life of the plant. In the absence of cooling water, bad things start to happen (can you say meltdown?), which I will get into later. For now, let’s examine this idea that the cooling system cannot be allowed to fail, ever.

Redundancy and more redundancy

First, we have to realize that the power for running the cooling pumps cannot come from the reactor itself. If it did, and you had to shut down the reactor, the turbine, or the generator for either maintenance or an emergency, then you’d immediately be into meltdown mode (no coolant circulation). So that doesn’t work. In our example, the power for the pumps comes from the national electrical grid. Unsurprisingly, highly-developed countries such as Japan or the US consider the grid to be a reliable source of power — except, of course, in the case of a major natural disaster. But TEPCO thought they had that covered, with sufficient diesel-powered generator capacity to run the pumps on all the reactors (and presumably the plant lighting and control functions). To back up the back-up, they also had battery capacity for 8 hours of operation. So far, so good.

An unanticipated sequence of events

It is unclear whether the earthquake itself knocked out power to the plant. Assuming it did, the pumps would have been running on power from the diesel generators when the tsunami hit. Leaving the tsunami aside, the “by the book” scenario would have had TEPCO emergency workers restoring power to the plant as a matter of priority. Given the scale of this particular earthquake, that may have taken days, but in the meantime trucks could deliver diesel fuel to keep the generators running. Potentially there could have been some delays involved with clearing the roads, but there was a certain amount of fuel storage on-site (I haven’t seen a figure for this, yet), and, in the worst case, they could go to batteries overnight. But, as we know, the earthquake was followed within an hour by a massive tsunami which not only drowned the diesel generators but probably severely damaged or destroyed the related infrastructure (power lines, control systems, fuel pipelines and storage tanks). By the following morning the battery power was exhausted, and the water in the reactors began to boil away.

Bad things start to happen

So, what are the adverse consequences of failing to circulate water through the reactor core? The first thing is the water starts to boil, creating steam. But the steam isn’t going anywhere (for instance, to power the turbine) because you can’t afford to let it flow out, since you don’t have any way of replacing the water (since the pumps are down). But that creates pressure, and eventually you have to release some of it to avoid blowing up the reactor vessel. So you are forced to vent some steam into the secondary containment structure. There are a few problems with this. One is that the steam contains traces of radioactive elements, but it’s not like you’re releasing it into the environment (not yet). The second is that the steam also contains hydrogen gas (the heat of the nuclear fuel breaks water vapor into hydrogen and oxygen; apparently this process is catalyzed or otherwise exacerbated by the zirconium casings on the fuel rods). Hydrogen gas is explosive (think Hindenburg disaster). Now the secondary containment is designed to deal with this situation — it is equipped with air pumps, filters and scrubbers that can capture the radioactive trace elements and the hydrogen gas — except for the fact that the power is out and none of this equipment is operational. Oops. The monitoring instruments that would inform the operators of a hydrogen build-up are also off-line.

Out of control

Up to this point nothing irreversible has occurred. If the power suddenly came back up, we’d soon be back to normal, with no permanent harm done. But unfortunately, events at Fukushima took a different turn. The first clue that something was seriously wrong at the complex occurred when one of the reactor buildings suffered a hydrogen explosion. The news reports stressed that this was not a nuclear explosion, and that the reactor containment was not breached. But what had happened was bad enough. The images clearly showed severe damage to the upper portion of the reactor building — gaping holes in the walls and roof, piles of twisted metal debris, and an ominous cloud of smoke. In my opinion, this was the critical event in the sequence of failures.

Prior to the explosion, full plant operation could have been restored by the flick of a switch. Now, it is impossible to say — it is conceivable that nothing can be done to prevent a full meltdown. This is a more pessimistic position than we have been hearing in the media, but I am finding it difficult to imagine how the situation can be quickly turned around. Look at the wreckage of the reactor building — even if TEPCO manages to hook up grid power, as they are attempting today, it seems unlikely that many of the building utility systems could remain functional. Lighting, control, monitoring and plumbing have all been massively damaged, at least insofar as the secondary containment and spent fuel storage functions are concerned (the reactor vessel and its systems can’t be seen). Debris removal, damage control, structural repair and systems replacement will have to be performed before full function can be restored, and all of this will necessarily be attempted in the dark (no lighting), in a structurally unstable environment choked with debris and subject to radiation leaks and further explosions, by exhausted workers hampered by rad suits and the absence of heavy equipment and power tools. My understanding is that this description applies to three of the reactor buildings as of Friday morning.

The situation of the plant workers is equivalent to that of a smoke jumper whose pumper has just been overrun by a fast-moving forest fire, leaving him standing downwind with a shovel and a bucket. He’s game, and he knows his business, but he doesn’t have anything to work with.

(to be continued)

This entry was posted on March 18, 2011 at 6:55 pm and is filed under Environment . You can follow any responses to this entry through the RSS 2.0 feed Both comments and pings are currently closed.